Substructures comprising one or more piles, either constituted by tubular sections or constituted by solid sections or even constituted by a combination of tubular sections and solid sections, also known as piled foundation, are known, mostly for establishing a foundation for wind turbines at sea, i.e. offshore. Such substructure consists of a primary structure and a secondary structure.
The primary structure is the structure performing load-bearing of the wind turbine itself. The primary structure is also the structure taking up loads of a magnitude comparable to the load of the wind turbine itself such as loads from waves, sea currents, earth tremors and other sea or ground-induced loads, loads from bending torques by the wind on the rotor of the wind turbine or loads from avalanches, stone slides or other such environmental loads.
The secondary structure is not performing load-bearing of the wind turbine itself or of the other loads mentioned above. The secondary structure is performing load-bearing of different servicing equipment to be used during installation of the foundation of the wind turbine and to be used during the operational life-time of the wind turbine. Such equipment may be boat-landings, access ladders, platforms, J/I-tubes, cranes, navigation aids, cathodic protection system, etc.
Secondary structures are mounted to the one or more tubular sections by means of fittings such as doubler plates. The fittings are often welded to the tubular section. Welding to the tubular section is time-consuming, but may also be devastating to the mechanical integrity of the foundation, both during establishment of the foundation and during the subsequent service life of the foundation.
Another known solution incorporates a mono-pile with a grouted transition piece. The transition piece is part of the primary structure. All secondary structures are welded or mounted to the transition piece. However, disadvantages of such a known solution are numerous. An additional lift for the transition piece is needed during installation. In case of deep water, either below water attachments (for e.g. J/I-tubes and cathodic protection system) must be attached to the mono-pile after driving the mono-pile into the ground, which involves expensive diver work and requires doubler plates welded to the mono-pile, or the transition piece must be extended to the seabed which increases the risk of a failing grout sealing. The grouted connection must be hardened before the wind turbine can be installed, which may take several days.
Therefore, the number of fittings such as doubler plates and the position of the fittings and the welding seams of such fittings have to be carefully selected, dimensioned and established in order to avoid impediment of the mechanical integrity of the tubular section. On the other hand, the fittings must be positioned so that the fittings are capable of supporting the secondary structure at the right level, at a sufficient basis and at locations where access to the fittings is possible.
U.S. Pat. No. 7,198,453 discloses a pile-based structural support device including a number of legs used to support a wind turbine. The wind turbine includes a base, a turbine generator and a blade mechanism. The legs are configured in a tee-pee type configuration such that the footprint of the base is larger than the footprint of the opposing end. This structural support can be used as a base for an offshore platform in that the support reduces the lateral forces on the support caused by wave action. Fittings are mounted to the legs of the tee-pee type configuration.
EP 1 770 276 discloses a method for installing an offshore wind turbine system which includes driving a cylindrical annular mono-pile into the ground. The mono-pile includes a flanged portion configured to support a wind turbine tower. The flanged portion extends radially from a peripheral surface of the mono-pile. The method further includes mounting a wind turbine tower directly onto the mono-pile, and the wind turbine is supported by the flanged portion of the mono-pile. No secondary structures are shown as being part of the foundation.
Thus, unfortunately, design and material considerations cannot be excluded regarding fittings being mounted to the tubular section, preferably by welding.
Also, when mounting the secondary structure to the tubular section, there is a risk of damage to any coating such as anti-corrosive paint on the tubular section.